This invention relates generally to signal processors and more particularly to real time signal processors which are adapted to process chirp or linear frequency modulated pulses.
As is known in the art, chirp or linear frequency modulated pulses are used in a variety of applications including pulse compression radars, as described in U.S. Pat. No. 4,028,700, inventors Carey et al., issued June 7, 1977, and assigned to the same assignee as the present invention, and frequency spectrum analyzers, as described in U.S. Pat. No. 4,005,417, inventor John D. Collins, issued Jan. 25, 1977, and assigned to the same assignee as the present invention. In such applications, the chirp pulse is pulse compressed in a matched filter to provide an indication of a target or the Doppler velocity of a moving target, for example. As is further known, the compressed pulse possesses undesirable sidelobes which can severely limit target or Doppler frequency resolution when the relative magnitudes of signals are large. Various weighting and equilization techniques to reduce such sidelobes are discussed in "Radar Handbook," M. I. Skolnick, Editor-In-Chief, McGraw-Hill Book Company, New York, N.Y. (1970), pages 20-26 to 20-35. Such techniques include various amplitude weighting techniques, such as Hamming weighting, and have been found satisfactory in many applications.
However, in many applications, it is necessary to separate the compressed pulse into "in phase" and "quadrature" components (see, for example, page 20-21 of the above-referenced "Radar Handbook"), as, for example, in radar system applications where the "sense" of the Doppler frequency is necessary, as in determining whether a target is "approaching" or "receeding" (i.e., has a "positive" or "negative" Doppler frequency). Such separation is conveniently performed by passing intermediate frequency compressed pulse signals through a quadrature phase detector to produce a pair of compressed video frequency pulses while preserving the phase of the target return (the rate of change of phase from pulse to pulse being proportional to the Doppler frequency of the target). However, while such technique is generally satisfactory where the chirp pulse has a relatively high time-bandwidth product (i.e., say in the order of 100), in applications where the chirp pulse has a relatively small time-bandwidth product, separation of the intermediate frequency compressed pulse into quadrature channels using such quadrature phase detection technique produces video frequency compressed pulses which retain relatively large sidelobes despite the use of the weighting techniques described above.